1. Field of the Invention
The present invention relates to an image and code data compression apparatus formed of an image data compression apparatus and a code data compression apparatus. More particularly, the image data compression apparatus is provided for compressing the image data in accordance with an orthogonal transformation and obtaining code data after compression of the image data, and the code data compression apparatus is provided for further compressing the code data in accordance with the orthogonal transformation and obtaining re-compressed code data after compression of the code data.
The image and code data compression apparatus according to the present invention can realize a high data compression rate in accordance with an algorithm for a compression method which is defined by JPEG (Joint Photographic Experts Group).
2. Description of the Related Art
As a method for compressing image and code data using an orthogonal transformation, an algorithm for compression was standardized by JPEG which was jointly organized by the International Standardization Organization (i.e., ISO) and the Comite Consultatif International Telegraphique et Telephonique (i.e., CCITT).
Such an algorithm is described, for example, in "JPEG, DIS 10918 DIGITAL COMPRESSION AND CODING OF CONTINUOUS-TONE STILL IMAGES, PART I REQUIREMENTS AND GUIDELINES". According to this document, image data are divided into a plurality of image data blocks, and each image data block is processed in accordance with a known DCT (Discrete Cosine Transformation) to calculate a DCT coefficient (below, transformation coefficient). Further, the calculated transformation coefficients are quantized to obtain quantized transformation coefficients. Still further, the quantized transformation coefficients are processed, in order, from a low frequency to a high frequency in accordance with a variable length coding process to realize compression of the image data.
In this case, the variable length coding process is performed for data which sums up the number of the quantized transformation coefficients having nonzero value and the number of the quantized transformation coefficients continuing just before the nonzero value.
When improving the above conventional method of image data compression which complies with the JPEG standard, basically, it is necessary to ensure the code data which can decode the image data by using a standardized decoding algorithm to realize an improved image data compression method.